1. Field of the Invention
The present invention relates to an improvement in a high pressure sodium vapor discharge lamp which comprises a translucent gas-discharge tube having therein sodium, buffer gas and xenon gas.
2. Description of the Prior Art
Generally speaking, high pressure sodium vapor discharge lamps have about a twice high efficiency in comparison with the conventional high pressure mercury lamp and have a comfortable color appearance, and therefore are attracting attention as energy saving light sources to replace the high pressure mercury lamp in the near future. In a discharge tube of a conventional high pressure sodium vapor discharge lamp, xenon gas is sealed to have a pressure of about 20 Torr, as a starting gas having a low thermal conductivity. As a result of the addition of xenon gas, a starting voltage of the high pressure sodium vapor discharge lamp is as high as about 2 KV, which is very high in comparison with less than 200 V of the mercury lamp. Recently, a high efficiency high pressure sodium vapor discharge lamp has been proposed, wherein xenon is filled with a pressure of over 150 Torr, for example, 350 Torr, thereby raising lamp efficiency by about 10% higher than those having xenon gas filled with a pressure of about 20 Torr, has been proposed. In such a lamp with high pressure xenon gas, the starting voltage of the lamp is further raised, for example, to about 8-9 KV for a high pressure sodium vapor discharge lamp having xenon gas filled with the pressure of 350 Torr. In such a lamp, the starting voltage can be lowered to about 3.5 KV for a 360 W high pressure sodium discharge lamp when utilizing a starting aid wire, but such voltage of 3.5 KV is still higher for actual use and liable to induce breakdown of insulation of a ballast and a related lamp circuit.